1. Field of the Invention
The present invention results to the low viscosity, broad range nematic media with large birefringence and moderately large dielectric anisotropy. The nematic media are useful in a variety of different electrooptical devices, including electronic lenses.
2. Background of the Technology
Liquid crystal (LC) media are used as dielectrics in different displays and devices, because of their optical properties which can be affectively affected on by applied electric field. Various electrooptical modes such as Twisted Nematic (TN), Supertwisted Nematic (STN), Super Birefringence (SBE), In-Plain Switching (IPS), Vertical Alignment Nematic (VAN) and Fringe Field Switching (FFS) are used and they need a LC medium with properties well tailored to the electrooptical effect and to the cell gap. The most important parameters that ensure good performance of displays and devices are birefringence Δn=ne−no (where ne and no are extraordinary and ordinary indices measured at direction parallel to the long and short molecular axis respectively), viscosity (especially rotational viscosity), elastic constants and dielectric anisotropy (the difference of electric permittivity constants ∈∥ and ∈⊥ measured at direction parallel to the long and short molecular axes, respectively).
The birefringence is the most important parameter for many devices, because it determines contrast and speed. A higher birefringence reduces LC crystal layer breadth (cell gap), and increases the response time.
For example, the transmission of light in the device using a twisted nematic layer comprising positive dielectric molecules (TN mode) which is still beneficially used both in direct and active matrices fulfills the expression (1):d·Δn=√{square root over (m2−1)}λ/2  (1)wherein d—cell gap, Δn—optical birefringence, λ—light wavelength, m=2 or 4 constant ensuring the minimum of light transmission.
The switching time being a period of time necessary to reorient molecules from the bright state (transmissive state) to the dark state (non-transmissive state) is given by the expression (2):
                              t          on                =                                            γ              1                        ·                          d              2                                                          π              2                        ·                                          k                11                            ⁡                              (                                                                            V                      on                      2                                        /                                          V                      th                      2                                                        -                  1                                )                                                                        (        2        )            wherein ton—rise time, γ1—rotational viscosity, k11—splay elastic constant, Von—applied voltage, Vth—threshold voltage.
The expressions (1) and (2) evidence that Δn values, as well as the ratio γ1/k11, are fundamental parameters of a LC medium that most affect the response time.
Higher dielectric anisotropy decreases the threshold voltage Vth, and for the fixed driven voltage Von the rise time ton may also be reduced, but the conductivity of the systems can also increase what is not accepted in many cases. Rather, materials that have low conductivity and moderate Δ∈ are the most desired.
Electronic lenses may be given as another example of devices wherein the birefringence of the LC medium used is very important, and widens the focal length during electric field change, see the formula (3),
                    f        =                              r            2                                              2              ·              d              ·              Δ                        ⁢                                                  ⁢            n                                              (        3        )            wherein f—focus length, r—lens radius, d—cell gap.
In this case, a LC composite medium is often used (see: Appl. Phys. Lett., 82 (9), 3537-3539 (2002)). The best performance for such an electronic lens is obtained when the material constants of the LC medium are kept at fixed range of values, for example such as: Δ∈=5-12, ∈⊥=18-30, γ1<200, Δn˜0.25-0.3 and nematic range is broader then −20 to 90. The properties of individual compounds such as: good chemical and photochemical stability, low melting point, low melting enthalpy, and low tendency to create a smectic phase, are also very important and determine their utility for being used as the component of the LC medium.
The cyano derivatives of biphenyl and terphenyl compounds are nematics with high birefringence, but their dielectric anisotropy and viscosity is rather large. See, for example, the properties of the mixture E7, which is a four component polar LC medium (Δn=0.225, η=40 cP)                Cr—N (−10); N-Iso 60.5; Δn=0.225; η=40 at 20° C.        (BDH Chemical Ltd., product information).Some LC crystals with isothiocyanato [NCS] terminal group have bigger birefringence, lower viscosity and dielectric anisotropy, and bigger elastic constant than cyano compounds, see U.S. Pat. No. 4,528,116 and paper Mol. Cryst. Liq. Cryst., 91, 17-27 (1990) but the isothiocyanato-biphenyl and terphenyls show only high ordered smectic phases, see example:        
                Cr 57.8 SmE 72.4 Iso Phase Transitions, 79, 331-342 (2006)Therefore they are not able to be used as the main component of the nematic mixtures. Fluorine atoms introduced to the biphenyl core cancel smectic phases and a nematic phase is created, but only as the monotropic one with low clearing point, see, for example:        
                Cr 31.8 (N 19.8) IsoPlacing into terphenyl core only one fluorine atom, still leads to the smectic compounds with very high melting points, see, for example:        
                Cr 168.2 SmA 194.7 N 226.5 Iso Conference Proceedings, XVI Conference on Liquid Crystals, Stare Jablonki 2005, Poland, s. 71-76 (2007)Isothiocyanatoterphenyls with two or more fluorine atoms in the aromatic core are nematics in broad temperature range. Their usefulness in preparing high birefringence nematic mixtures was recently demonstrated in Opto-Electronics Review 15(1), 47-51 (2007), but they have simultaneously large dielectric anisotropy (Δ∈>10), such as mixtures of cyanocompounds. Among isothiocyanato derivatives with cyclohexylphenyl, bicyclohexylphenyl and cyclohexylethylcyclohexylphenyl cores the lowest kinematic and rotational viscosity and high k11 values shown the following compounds:        
                n=6 Cr 12.5 N 43.0 Iso, U.S. Pat. No. 4,528,116 (1985)        
                n=3 Cr 76.5 N 248.7 Iso, Liq. Cryst. 26, 1817-1823 (1999)        
                n=4 Cr 87.5 N 260 Iso, Eur. Pat., 0 272 580and the mixture with low viscosity and low temperature dependence of viscosity may be created, see U.S. Pat. Nos. 4,849,130, 4,528,116, and paper SPIE, 4147, 41-48 (1999).        
Because of the presence of saturated rings they have lower birefringence than fully aromatic structures.
Although mixtures composed of simultaneously cyclohexylphenyl derivatives of isothiocyanates and some amounts of smectic biphenyl derivative of isothiocyanates and nematic terphenyl isothiocyanates may show birefringence higher than 0.25, but still their dielectric anisotropy is too high for some applications. Usually to decrease the dielectric anisotropy of the LC medium it is necessary to dilute it with low polar or nonpolar liquid crystalline compounds. The nonpolar high birefringence hydrocarbons, such as dialkyl- or alkyl alkoxy-, biphenyl or terphenyl have only high ordered smectic phases, see for example:
                Cr 25.1 SmE1 46.1 SmE2 47.1 N 52.3 Iso Mol. Cryst. Liq. Cryst., 260, 435-442 (1995)Those which have a nematic phase, such as esters or tolanes, which are frequently and commonly used as the components of high birefringence mixtures, are not useful here because together with the isothiocyanates they have strong tendency to induce smectic phases Mol. Cryst. Liq. Cryst., 124, 241-257 (1985).        
Accordingly, finding good components for decreasing dielectric permittivity of isothiocyanato nematic mixtures without strongly decreasing their birefringence, and keeping their nematic phase over a broad range of temperatures is still a problem.